Tomasz Szynalsky's Online Tone Generator
Among free online tone generators, this one was able to produce the purest tones we found. Change frequency with a slider or by clicking the table of musical notes. You can produce highly precise frequencies up to 0.001 Hz.

Physics Toolbox Sensor Suite
This free app plays on all major devices in Android and iOS. The tools applicable to the topic of Waves & Optics include oscilloscope, sound meter, audio spectrogram, light meter, color detector, tone generator, and spectrum analyzer.

Academo: Spectrum Analyzer
This fascinating free tool lets you choose from pre-set audio files including an Orca whale, songbird, police siren, and musical instruments, or upload your own. The signal is generated by applying a Fourier transform to the time-domain signal. This produces a graph of all the frequencies present in a sound recording at a given time.

Academo: Virtual Oscilloscope
With this online oscilloscope, you can use your own microphone to input live sound and see the resulting waveform. Numerous tools include an "input freeze" button, frequency and voltage settings, oscilloscope gain slider, and ability to view as a sine or square wave. Works on all major devices; mobile-friendly.

GEAS Project: Spectral Analysis Tools
Students can use authentic data to explore emission/absorption spectra to determine presence of elements, look at active galactic nuclei (AGN) spectra, and analyze stellar spectra for different spectral types. Tools allow for detailed line-fitting profiles and measurement. Blackbody curves can be fit to continuum levels to determine temperatures.

Online Tone Generator
This free resource opens in all major browsers. It offers numerous tools to explore sound without expensive mechanical frequency generators. Just enter the desired frequency and click Start. You can generate 2 frequencies at once, create beats, and check your own hearing.